Keyless access systems are used e.g. in motor vehicles to impede the access to or entry into the vehicle of unauthorized persons, or even to entirely prevent authorized access. On the other hand, access for authorized persons is to be made as easy as possible as is the case when using conventional keys.
Document EP 0 570 761 B1, incorporated by reference herein, has disclosed a passive keyless access system. The latter comprises a portable signal generator which has a sensor for transmitting an encoded generator signal. The encoded generator signal contains a clock code with a receiver assigned to the vehicle. The receiver has an antenna for receiving the encoded generator signal when the portable signal encoder is within a specific range around the vehicle. The receiver contains control means for decoding the generator signal and activating specific functions, specifically the opening and closing of the vehicle doors.
In such keyless access systems, it is necessary for some functions to ascertain the distance of the signal generator relative to the vehicle. It is possible to carry out certain functions only when the signal generator is at less than a specific limiting distance. Such functions can be, for example, a comfort closing feature of windows and/or a sunroof of the motor vehicle using the signal generator. For reasons of safety, the limiting distance is selected such that it is ensured that the person activating the signal generator observes the vehicle.
In the case of keyless access systems which operate with an LF frequency of 125 or 130 kHz, sufficient precision during the determination of the distance of the signal generator relative to the vehicle is possible by processing a measured field strength since the LF field strength is dependent to the third power on the distance.
Since cell phones are becoming increasingly widespread, it is desirable to use them as a signal generator for a keyless access system. The passive access to the vehicle can be carried out e.g. by means of a short-range radio link, for example on the basis of the Bluetooth Standard, between the vehicle and the cell phone. Owing to the relatively high frequencies in the gigahertz range and the propagation as an electrical field, it is not possible to ascertain the distance between the cell phone as a signal generator and the vehicle with sufficient accuracy through the determination of the RF field strength (RF, RSSI—received signal strength indicator). This is due to the fact that the RF field strength is dependent to the second power on the distance.